A. Field of the Invention
The invention is in the field of x-ray detector systems as used in x-ray producing instrumentation such as electron microscopes, microprobes, x-ray spectrometers, etc. In particular, the invention relates to a device for improving the solid angle of x-ray collection while still providing uncompromised protection for the x-ray detector crystal under conditions of damaging radiation.
B. Related Art
In a system which utilizes an energy dispersive x-ray detector system, such as an electron microscope, a sample is bombarded with an electron beam of up to several hundred thousand electron volts. X-rays and secondary electrons fluoresced by the sample fall incident on the x-ray detector crystal placed in close proximity to the sample. Ideally, the detector needs to be as close as geometrically possible to the sample to obtain the best solid angle of x-ray collection. However, the x-ray detector needs to be protected when excessively high x-ray or secondary electrons are being produced. This occurs, when, for instance, a Transmission Electron Microscope is operating in low magnification mode or when the probe beam is allowed to fall incident on an object which produces high energy x-ray fluorescence. Under these conditions, very high signal currents pass through the detector crystal which causes the electronics in the system to reset at high frequency. This condition may result in detector crystal or electronic component damage.
In the past, when the current in the detector was high, a shutter was placed between the detector crystal and the sample. An example of the prior art appears in British patent GB 2192091A. This prior art has the disadvantage of requiring some distance between the sample and the x-ray detector for the shutter mechanism. Having the detector fixed behind a shutter, which opens or closes, significantly compromises the solid angle of x-ray collection during analysis.
In U.S. Pat. No. 4,450,355, a thin shutter mechanism was controlled by the same magnetic field that controlled the magnification lens. In high magnification, the high magnetic field opens the shutter allowing the detector to measure x-rays from the sample. In low magnification, the shutter remains closed preventing most of the high energy secondary electrons from entering from the detector. This prior art had the disadvantage that there was not sufficient protection of the detector in practice. In particular, when the magnetic field was at some intermediate levels, the shutter stayed open and electrons got to the detector. Also the relatively thin shutter, a fraction of a millimeter of brass, did not adequately stop the very high energy x-rays.
Moreover, in order to allow for space for the shutter mechanism in both of the above patents, the detector had to be positioned at too great a distance from the sample to permit optimal detection.